Beta-adrenergic receptor subtypes in stress-induced behavioral depression

The purpose of this study was to examine the role of beta-adrenergic receptors in an animal model of stress-induced behavioral depression. beta-Adrenergic receptors in several brain regions and leukocytes of rats were determined by receptor binding techniques using 125I-cyanopindolol (cyp) as ligand and propranolol as displacer for total beta-adrenergic receptors, and ICI 86,406 for beta 1- and ICI 118,551 for beta 2-adrenergic receptors. We observed that the maximum number of binding sites (Bmax) and the apparent dissociation constant (Kd) of 125I-cyp binding to total beta-adrenergic receptors were increased in hippocampus of stressed rats with escape deficits (48 h after training) as compared to control rats. This increase was due to an increase in Bmax and Kd of 125I-cyp binding to beta 1-adrenergic receptors but not to beta 2-adrenergic receptors. There was no significant difference in beta 1-adrenergic receptors in cortex and cerebellum or beta 2-adrenergic receptors in hippocampus, cortex, cerebellum, or leukocytes of stressed (48 h after training) rats with escape deficits as compared to control rats. Interestingly, it was observed that beta 1- and beta 2-adrenergic receptors in various brain regions (cortex, cerebellum, and hippocampus) and beta 2-adrenergic receptors in leukocytes of stressed rats (10 days after training) were not significantly different from control rats, although escape deficits were still present. These results suggest that abnormalities in adrenergic neurotransmission are associated with an upregulation of beta 1-adrenergic receptors, which in turn may be involved in the early stages of behavioral deficits caused by uncontrollable shock.

Characteristics of the beta-adrenergic receptor complex in the epicardial border zone of the 5-day infarcted canine heart

BACKGROUND

The effect of isoproterenol on increasing the peak amplitude of the L-type calcium current is reduced in myocytes dispersed from the epicardial border zone (EBZ) of the 5-day infarcted canine heart when compared with control cells from noninfarcted hearts. This suggests that specific alterations in the beta-adrenergic receptor complex develop in this setting. The present study is an examination of individual components of the beta-adrenergic receptor complex with the aim of elucidating the biochemical defect(s) that might be responsible for the diminished beta-adrenergic receptor responsiveness in the myocytes that survive in the infarcted heart.

METHODS AND RESULTS

We compared components of the beta-adrenergic receptor signaling pathway in membranes prepared from the EBZ of the 5-day infarcted heart and a remote, noninfarcted region (RZ) of the same ventricle as well as the corresponding regions of noninfarcted ventricles. Defects in multiple components of the beta-adrenergic receptor complex were confined to the EBZ of the 5-day infarcted heart. These include a decrease in beta-adrenergic receptor density; diminished basal, guanine nucleotide-, isoproterenol-, forskolin-, and manganese-dependent adenylyl cyclase activities; an increase in the EC50 for isoproterenol-dependent activation of adenylyl cyclase; a diminished level of the alpha-subunit of the Gs protein. and an elevated level of the alpha-subunit of the Gi protein.

CONCLUSIONS

Defects in multiple components of the membrane beta-adrenergic receptor complex were identified in the EBZ of the 5-day infarcted canine heart. This constellation of abnormalities would be predicted to impair functional beta-adrenergic responsiveness and contribute to the defect in isoproterenol-dependent stimulation of the L-type calcium current in myocytes isolated from this tissue.

Alterations in beta-adrenoceptors and polyploidy in cultured aortic smooth muscle cells from different age groups of spontaneously hypertensive rats and Wistar-Kyoto rats

OBJECTIVE

The relationship between the number of beta-adrenoceptors and polyploidy in cultured aortic smooth muscle cells derived from different age groups of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats were examined.

DESIGN

The number of beta-adrenoceptors, the percentage of multinucleated cells and the incidence of polyploidy from cultured smooth muscle cells derived from SHR and WKY rats aged 3-4, 10-12 and 28-30 weeks were measured. The effect of passaging of the cells on the expression of beta-adrenoceptors and polyploidy on cultured smooth muscle cells from both SHR and WKY rats was also investigated.

METHODS

Receptor binding experiments were carried out using [125]-monoiodocyanopindolol with osmotically lysed cultured aortic smooth muscle cells to investigate the properties of vascular beta-adrenoceptors in SHR and WKY rats. The proportion of polyploid smooth muscle cells was determined by frequency distribution analyses of Feulgen DNA microdensitometric measurements.

RESULTS

The incidence of polyploid smooth muscle cells was consistently higher in cells cultured from SHR than in those from WKY rats in all three age groups, with a positive correlation between polyploidy and age in SHR. Furthermore, in all three age groups the number of beta-adrenoceptor binding sites was also higher in cultured smooth muscle cells from SHR than in those from WKY rats. There was no significant difference in the receptor affinity. The increase in beta-adrenoceptor number was associated with an increase in polyploidy, and both of these changes were positively correlated both with the age of the rats from which these cells were derived and with the number of passages.

CONCLUSIONS

Under cell culture conditions the expression of beta-adrenoceptor density increases with the number of passages in both SHR and WKY rats. Smooth muscle cells derived from older SHR and WKY rats have a greater propensity to develop polyploidy. This trend is significantly accelerated in cultured smooth muscle cells derived from SHR compared with those from WKY rats, suggesting a premature ageing process. These findings suggest that, in cultured smooth muscle cells from SHR and WKY rats, beta-adrenoceptors may influence the expression of polyploidy.

Hypertrophic cardiomyopathy: a desensitized cardiac beta-adrenergic system in the presence of normal plasma catecholamine concentrations

Only few data are available concerning the biochemical and functional state of the beta-adrenergic system in hypertrophied human myocardium. The present study was to investigate the myocardial beta-adrenergic signal transduction system in hypertrophic obstructive cardiomyopathy (HOCM). Thin myocardial strips were prepared from surgically excised, septal myocardium from 7 patients with HOCM and their force of contraction was measured in vitro. The positive inotropic effects of calcium and dihydro-ouabain, both acting independently of beta-adrenoceptors and cAMP, were similar in these preparations to those, previously published, seen with nonfailing myocardium. In contrast, the beta-adrenoceptor agonist isoprenaline and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) had reduced positive inotropic effects. Their EC50-values were about 10 fold higher than the respective EC50-values published for nonfailing myocardium. The positive inotropic potencies of isoprenaline and IBMX were reduced in HOCM by as much as they were in the additionally investigated myocardium from 6 patients with severe mitral regurgitation (MR, NYHA III). In order to clarify whether the functional alterations are related to changes in the beta-adrenoceptors, beta-adrenoceptor density and beta 1: beta 2-adrenoceptor subtype distribution were determined in the same myocardium using 125I-Iodocyanopindolol saturation binding. Myocardial beta-adrenoceptor density was reduced to 68% in HOCM and to 56% in MR compared to nonfailing myocardium controls (NF: 64.8 +/- 6.5 fmol/mg protein). In HOCM, this reduction was due to a selective down regulation of beta 1-adrenoceptors (24.9 +/- 3.7 fmol/mg protein vs NF: 46.4 +/- 6.8 fmol/mg protein, P < 0.05), whereas beta 2-adrenoceptor density was unchanged (19.0 +/- 1.9 fmol/mg protein vs NF: 18.4 +/- 3.3 fmol/mg protein, n.s.). In MR both beta-adrenoceptor subtypes were reduced (beta 1: 26.9 +/- 1.4 fmol/mg protein, beta 2: 9.6 +/- 1.7 fmol/mg protein; both P < 0.05 vs NF). Electrochemically determined plasma catecholamine levels were elevated in MR. However, plasma catecholamine levels were normal or slightly below normal in HOCM. In summary, myocardial beta-adrenoceptors are downregulated and their function is impaired in HOCM. This desensitization is not caused by a negative feedback regulation due to increased plasma catecholamines. The present results show that the desensitizations of the beta-adrenergic system associated with HOCM has characteristics that indicate a major deviation in its development from that of the beta-adrenergic desensitization previously described to occur in congestive heart failure.

Beta-adrenoceptor-G alpha S coupling decreases with age in rat aorta

beta-Adrenoceptor (beta AR) responsiveness, receptor density, receptor-G protein coupling, and the possible role of membrane fluidity in receptor-G protein coupling were investigated in the rat aorta with age. The beta AR agonist isoproterenol (ISO) produced relaxation of KCl-induced aortic contractions by 97%, 21%, and 0% in aortae from 1- 6-, and 24-month-old Fischer 344 rats, respectively. Forskolin completely relaxed the contractions at all ages. beta AR density was determined in aortic membranes by saturation binding of 125I-cyanopindolol (125I-CYP). beta AR density was 76, 52, and 47 fmol/mg of protein in 1-, 6-, and 24-month-old rats, respectively. To investigate beta AR coupling to G proteins, displacement by ISO of 125I-CYP binding was determined in aortic membranes in the presence and absence of the GTP analog guanosine-5'-(beta gamma-imido)triphosphate [Gpp(NH)p] (0.1 mM). The effect of Gpp(NH)p on the ISO displacement curve for 125I-CYP binding was greatest in 1-month-old rats and decreased markedly with age. In 1-month-old aorta, in the absence of Gpp(NH)p the ISO displacement curve was biphasic and two affinity constants were determined (KH - 0.061 microM and KL = 2.4 microM). In the presence of Gpp(NH)p the ISO displacement curve was monophasic (Kd - 0.72 microM). In 6-month-old aorta, whereas an effect of Gpp(NH)p on the ISO displacement curve could still be observed [in the absence of Gpp(NH)p, KH = 0.2 microM and KL = 3.5 microM; in the presence of Gpp(NH)p, Kd - 0.83 microM], the affinity constant for high affinity agonist binding and the percentage of receptors with high affinity for agonist were decreased significantly. In 24-month-old aorta there was no effect of Gpp(NH)p on the ISO displacement curve and a single affinity constant was detected [0.7 microM and 0.8 microM in the presence and absence of Gpp(NH)p, respectively]. The presence of two affinity constants for ISO in 1- and 6-month-old aorta in the absence of Gpp(NH)p and single affinity constants in the presence of Gpp(NH)p presumably represent the G protein-coupled and uncoupled states of the beta ARs, which are not observed in 24-month-old aorta. The ability of the beta AR to form the high affinity nucleotide-sensitive complex with the agonist was restored by treatment of the membranes with cis-vaccenic acid, which increases the fluidity of the membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Carboxyl-terminal domains in the avian beta 1-adrenergic receptor that regulate agonist-promoted endocytosis

Most G protein-coupled receptors, including the mammalian beta 2-adrenergic receptor, are endocytosed to an intracellular, vesicular compartment upon continued exposure to agonist. The long form of the avian beta 1-adrenergic receptor, which contains a carboxyl-terminal 59-amino acid extension, does not undergo agonist-promoted endocytosis. We constructed and expressed turkey beta 1-adrenergic receptor cDNAs with regularly spaced carboxyl-terminal truncations and studied their agonist-promoted endocytosis. Removal of 34-86 amino acids from the carboxyl terminus of the turkey receptor allowed its efficient endocytosis, with optimal endocytosis observed upon removal of 59 residues. Removal of only 18 residues allowed some endocytosis. A receptor that lacks the entire carboxyl-terminal region (124 residues) was not endocytosed. We also constructed a chimeric hamster beta 2-adrenergic receptor with the added 59-residue carboxyl-terminal domain of the turkey receptor. The chimera was not significantly endocytosed. These data indicate that residues 450-465 in the carboxyl-terminal region of the beta 1-adrenergic receptor can act independently to block agonist-promoted endocytosis and that other carboxyl-terminal structures nearer to the seventh membrane span are required for endocytosis.

Serotonin receptor binding in a colony model of chronic social stress

Male rats housed in mixed-sex groups quickly established dominance hierarchies in which subordinates appeared severely stressed. Subordinate rats had elevated basal corticosterone (CORT) levels relative to dominants and individually housed controls. Several subordinates had blunted CORT responses to a novel stressor, leading to the classification of subordinates as either stress-responsive or nonresponsive. Binding to 5-HT1A receptors was reduced in stress-responsive subordinates compared to controls throughout hippocampus and dentate gyrus. Decreased binding was observed in nonresponsive subordinates only in CA3 of hippocampus. In addition, 5-HT1A binding was decreased in CA1, CA3, and CA4 in dominants compared to controls. Binding to 5-HT2 receptors was increased in parietal cortex in both responsive and nonresponsive subordinates compared to controls. No changes were observed in binding to 5-HT1B receptors. These results are discussed in the context of regulation of the serotonergic system by stress and glucocorticoids and possible relevance to the pathophysiology of depression.

The development of beta 1-adrenoceptors in brown adipose tissue following prenatal alcohol exposure

Prenatal alcohol exposure delays the development of thermoregulation in newborn rats. Newborns generate heat by the sympathetic nervous system's activation of nonshivering thermogenesis in brown adipose tissue (BAT). In this study, the effects of prenatal alcohol exposure on the development of the beta-adrenergic receptor system of BAT was investigated by assessing the number and pharmacological properties of beta-adrenergic receptors in BAT in 1-, 5-, 10-, and 20-day-old offspring. Pregnant dams were given either a liquid diet with 35% of the calories derived from alcohol, a liquid diet without alcohol for any effects of the liquid diet administration, or ad lib food and water. Offspring from the alcohol prenatal treatment group had a greater number of beta 1 adrenergic receptors compared to offspring from both from the pair-fed and lab chow control groups, which did not differ from each other. The greater number of receptor sites in 5-day-old subjects suggests that the number of binding sites in alcohol-exposed BAT cells continues to rise due to the absence of sufficient neurotransmitter, and perhaps reflects a delay in the arrival of sympathetic nervous system neurons. During the second and third postnatal weeks, when NE concentrations are rising and reaching asymptotic levels, the number of beta 1 adrenergic receptors in BAT of control subjects is decreasing. This expected compensatory "downregulation" response in receptor concentration was not seen in BAT from subjects exposed to alcohol prenatally. These findings may have important implications for understanding the effects of prenatal alcohol exposure on developing plasticity in the peripheral nervous system.

Regulation of beta-adrenergic receptors in acute myocardial ischemia: subtype-selective increase of mRNA specific for beta 1-adrenergic receptors

Acute myocardial ischemia leads to a rapid increase of cardiac beta-adrenergic receptors in plasma membranes despite the release of large and desensitizing amounts of endogenous catecholamines. Part of this increase has been shown to occur at the expense of intracellular receptors. To investigate whether an additional expressional regulation of beta-adrenergic receptors due to an increase of mRNA levels is involved, the mRNA levels specific for beta 1- and beta 2-adrenergic receptors were determined after various periods of global ischemia in isolated perfused rat hearts. The subtype-specific quantification of mRNA for beta 1- and beta 2-adrenergic receptors was determined using reverse-transcription followed by PCR (RT-PCR) and RNA protection assays. RT-PCR resulted in single amplification products of the expected sizes (159 bp for beta 1-adrenergic receptors and 240 bp for beta 2-adrenergic receptors). The specificity of these amplification products was confirmed by specific restriction digests. Southern blot hybridizations with internal oligonucleotides and sequencing using the dideoxy chain termination method. For quantification purposes, the mRNAs of housekeeping gene GAPDH and of cardiac alpha-actin were determined as internal standards. Additionally, cRNAs specific for beta 1- and beta 2-adrenergic receptors were used as external standards. Brief periods of global ischemia induced a rapid increase in the steady state level of mRNA for beta 1-adrenergic receptors. There was a statistically significant rise already after 15 min by 57% compared to controls. After 30 min of ischemia the mRNA levels had almost doubled. After 60 min of ischemia, the mRNA levels specific for beta 1-adrenergic receptors tended to decrease, but remained significantly above normoxic controls. In contrast, the mRNA levels specific for beta 2-adrenergic receptors remained constant up to 60 min of global myocardial ischemia. To investigate, whether agonist occupancy of the receptors may contribute to this regulation, the effect of preperfusion with the beta-blocker alprenolol was determined. Contrary to expectation, beta-blockade did not influence the ischemia-induced increase of mRNA levels specific for beta 1-adrenergic receptors. These data demonstrate for the first time, that acute myocardial ischemia induces a rapid, and subtype-selective regulation of mRNA levels for beta 1-adrenergic receptors. However, occupation or activation of beta-adrenergic receptors by an agonist is not involved in this newly characterized regulation of mRNA for beta 1-adrenergic receptors in acute myocardial ischemia.

Adjustment of metabolism, catecholamines and beta-adrenoceptors to 90 min of cycle ergometry

Adrenaline infusion of 0.1 microgram.kg-1.min-1 in healthy volunteers results in an increase of hepatic glucose production, an increase of the absolute number of occupied beta-adrenoceptors and specific changes in metabolism. To compare these effects with the changes induced by an endogenous catecholamine release, we investigated healthy volunteers during cycle ergometry. After fasting at least 14 h seven healthy subjects exercised for 90 min at an intensity of 20% below their individual anaerobic threshold. The rate of glucose production as well as the turnover rates of alanine and leucine were calculated using stable isotope tracers. High and low affinity beta-adrenergic binding sites on lymphocytes were determined by an equilibrium binding assay with (-)125 Iodocyanopindolol. After 90 min of cycling the rate of appearance of glucose increased significantly from means of 2.0 (SD 0.2) to 2.65 (SD 0.50) mg.kg-1.min-1 with unchanged blood concentrations of glucose and lactate. The flux of the amino acids alanine and leucine decreased significantly from means of 0.91 (SD 0.21) to 0.62 (SD 0.14) mg.kg-1.min-1 and from 0.40 (SD 0.05) to 0.32 (SD 0.04) mg.kg-1.min-1, respectively. The mean free fatty acid concentration increased significantly from 0.65 (SD 0.33) to 1.27 (SD 0.45) mmol.l-1 during the endurance trial. The increase of glucose turnover and the decrease of amino acid flux point to a metabolic shift towards enhanced utilization of free fatty acids. Adrenaline and noradrenaline concentrations showed a moderate but significant increase from means of 0.61 (SD 0.20) to 0.99 (SD 0.36) nmol.l-1 and from 2.27 (SD 0.75) to 3.46 (SD 0.38) nmol.l-1, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

High beta-adrenoceptor density on peripheral blood mononuclear cells in progressive multiple sclerosis: a manifestation of autonomic dysfunction?

In multiple sclerosis (MS) up-regulation of beta-adrenoceptors on peripheral blood mononuclear cells (PBMCs) has been attributed to either autonomic dysfunction, inflammation or a combination of the two. We have compared secondary progressive MS patients with normal subjects (NS) and two models of autonomic dysfunction; pure autonomic failure (PAF) and multiple system atrophy (MSA, Shy-Drager syndrome). There was up-regulation of beta-adrenoceptors on PBMCs in MS and PAF patients but not in MSA patients. Only in PAF patients beta-adrenoceptor up-regulation was correlated with low plasma levels of noradrenaline (NA) and adrenaline (Ad). In addition to studies in the basal state, measurements also were made after the centrally acting sympatholytic agent clonidine. These were combined with haemodynamic and neurohormonal measurements. After clonidine, there was a fall in blood pressure in NS and MSA patients but not in MS and PAF patients; a rise in growth hormone (GH) in NS and PAF patients but not in MS and MSA patients; and an up-regulation in PBMCs beta-adrenoceptors in NS but not in MS, MSA and PAF patients. Up-regulation of beta-adrenoceptors on PBMCs in MS could be attributed to autonomic dysfunction but the disparity between MS and PAF patients when considering their plasma levels of NA and Ad argue against. Although the neurohormonal responses to clonidine and the physiological assessment of autonomic function in progressive MS patients, demonstrate central autonomic dysfunction resembling that of the MSA patients, the normal basal beta-adrenoceptor densities in the latter, suggests that the up-regulation of these receptors is independent of the central autonomic dysfunction in MS.

Modulation of adrenergic receptors during regression of cardiac hypertrophy

OBJECTIVE

To determine whether alpha 1- or beta-adrenergic receptors are altered during regression of cardiac hypertrophy produced by antihypertensive agents.

DESIGN AND METHODS

Cardiac hypertrophy was induced in rats by aortic banding. After 6 weeks banding the rats were treated with an angiotensin converting enzyme (ACE) inhibitor (enalapril), an alpha 1-adrenergic antagonist (bunazosin) or a beta-adrenergic antagonist (propranolol) for 6 weeks to induce regression. The numbers of alpha 1- and beta-adrenergic receptors, haemodynamics, tissue noradrenaline content and tissue ACE activity were measured.

RESULTS

Regression of cardiac hypertrophy occurred after treatment of aortic banded rats with a high dose of enalapril, bunazosin or propranolol, and was accompanied by a reduction in systolic blood pressure. The number of alpha 1- or beta-adrenergic receptors was unchanged by propranolol treatment, but the number of alpha 1-adrenergic receptors was increased in the hearts of rats treated with bunazosin. A low dose of enalapril (3 mg/kg body weight) caused regression of hypertrophy without a concomitant reduction in blood pressure, and decreased the number of alpha 1-adrenergic receptors. The dissociation constants for alpha 1- and beta-adrenergic receptors were not different among the experimental groups, and the positive derivatives of left ventricular pressure was unaltered in rats treated with a low dose of enalapril but was reduced by the other drugs.

CONCLUSION

Of the three drugs tested, only the low dose of enalapril affected adrenergic receptors during regression of cardiac hypertrophy, causing a decrease in alpha 1-adrenergic receptor number without a reduction in blood pressure. This effect may be explained by non-haemodynamic actions of the ACE inhibitor enalapril, probably by modulation of peripheral sympathetic activity.

Age-related changes in beta-adrenergic neuroeffector systems in the human heart

BACKGROUND

Aging decreases cardiac beta-adrenergic responsiveness in model systems and in humans in vivo. The purpose of this study was to comprehensively evaluate the age-related changes in the beta-receptor-G protein-adenylyl cyclase complex in nonfailing human hearts.

METHODS AND RESULTS

Twenty-six nonfailing explanted human hearts aged 1 to 71 years were obtained from organ donors and subjected to pharmacological investigation of beta-adrenergic neuroeffector systems. When the population was subdivided into the 13 youngest and 13 oldest subjects, total beta-receptor density assessed by maximum [125I]ICYP binding (beta max) was reduced in older hearts by 37% in left ventricles and 31% in right ventricles (both P < .05), and the downregulation was confined to the beta 1 subtype (r = .78 left ventricle beta 1 density versus donor age). Older donor hearts exhibited a 3- to 4-fold rightward shift of ICYP-isoproterenol (ISO) competition curves and demonstrated 43% fewer receptors in a high-affinity agonist binding state (P < .05). Older hearts exhibited decreased adenylyl cyclase stimulation by ISO, by zinterol (beta 2-agonist), and by the G protein-sensitive probes forskolin, Gpp(NH)p, and NaF. In contrast, there was no change in response to manganese, a specific activator of the adenylyl cyclase catalytic subunit. Toxin-catalyzed ADP ribosylation in membranes prepared from older versus younger hearts revealed a 29% to 30% reduction (P < .05) with cholera toxin (Gs) but no difference with pertussis toxin (Gi). The systolic contractile response of isolated right ventricular trabeculae to ISO was decreased by 46%, with a 10-fold increase in ISO EC50 in older relative to younger donor hearts.

CONCLUSIONS

There is a profound decrease in cardiac beta-adrenergic responsiveness with aging. This occurs by multiple mechanisms including downregulation and decreased agonist binding of beta 1-receptors, uncoupling of beta 2-receptors, and abnormal G protein-mediated signal transduction.

Antagonism between T3 and amiodarone on the contractility and the density of beta-adrenoceptors of chicken cardiac myocytes

3,3',5-Triiodothyronine (T3), at 10(-8) M, potentiated by 26.4-30.9% the isoproterenol-mediated inotropic effect in chick embryo cardiac myocytes in culture. Amiodarone (10(-6) M) decreased this response by 44.6% only in cells cultured with serum, where the T3 concentration was 10(-13) M. Amiodarone inhibited the potentiating effect of T3. Amiodarone alone had no influence on the beta-adrenoceptor density in cells cultured in serum-free medium. This confirms that the effects of amiodarone on cardiac beta-adrenoceptors are T3 dependent. T3 increased the density of beta-adrenoceptors through two concentration ranges, with an initial 30% increase between 10(-14) and 10(-11) M, followed by a second increase until 10(-7) M. Amiodarone not only inhibited the first positive effect of T3 but also decreased beta-adrenoceptor density far below the control value. The second positive T3 effect was also inhibited by 50% by amiodarone. This study suggests that T3 might increase the number of cell-surface beta-adrenoceptors and modify their cellular traffic through at least two mechanisms, one assumed to be non-genomic, the other being genomic, and that amiodarone could affect the two mechanisms differently.

Effect of age on beta-receptors, Gs alpha- and Gi alpha- proteins in rat heart

beta-adrenoceptors, Gs alpha- and Gi alpha-proteins were investigated in a crude plasma membrane preparation from ventricles of young (2-4 months) and senescent (22-24 months) Wistar rats. Receptor density, ligand affinity and beta 1/beta 2-receptor ratio were independent of the age of the rats. The percentage of beta-receptors coupled to G-proteins increased with age. An age-related increase in the level of Gs alpha (124%) was paralleled by an increase in the ratio between the high and low molecular weight form of Gs alpha. The level of Gi alpha-protein almost doubled (170%) upon aging. We conclude that the age-related differences are small at the level of the beta-adrenoceptor molecule, but that the increase in Gi alpha-proteins could be responsible for the age-related reduction in myocardial inotropic and chronotropic responses. Moreover, we suggest that the changes in degree of high affinity coupling between beta-receptor and Gs-protein are possibly linked to alterations in the ratio between the Gs-molecular weight subtypes.

Up-regulation of beta 1-adrenergic receptors in rat brain after chronic citalopram and fluoxetine treatments

Quantitative receptor autoradiography was used to study the effects of the selective serotonin reuptake inhibitors citalopram and fluoxetine and the tricyclic antidepressant imipramine on the regulation of beta 1-adrenergic receptors in the rat brain. Rats were treated with saline, citalopram (10 mg kg-1), fluoxetine (10 mg kg-1), or imipramine (15 mg kg-1) SC once daily for 14 days. [125I]Iodocyanopindolol binding to beta 1-adrenergic receptors was found to increase significantly in the caudate-putamen and the somatosensory areas of the frontal cortex after both citalopram and fluoxetine treatments. Imipramine treatment elicited a marked decrease in beta 1 binding in the outer laminae of the cingulate cortex, as well as in the motor and somatosensory areas of the frontal cortex. In a separate experiment, rats were treated with saline, citalopram (2.5, 10 and 20 mg kg-1) or fluoxetine (2.5, 10 and 20 mg kg-1) SC once daily for 14 days. The effects of citalopram and fluoxetine on beta 1 receptors in the somatosensory cortex and caudate-putamen were replicated. These results demonstrate that chronic administration of selective serotonin reuptake inhibitors, in contrast to imipramine, can cause a regional up-regulation of beta 1-adrenergic receptors in the rat brain.

Pertussis toxin pretreatment alters agonist binding to beta-adrenoceptors in rabbit ventricular myocardium

The influence of pretreatment with pertussis toxin (PTX) on characteristics of beta-adrenoceptor bindings was examined in myocardial ventricular membranes prepared from rabbits which received an intravenous injection of PTX (10 micrograms/kg) or its vehicle. The density and affinity of beta-adrenoceptors assessed by (-)-[125I]iodocyanopindolol were not significantly altered by PTX pretreatment. However, a fraction of agonist binding sites with high affinity significantly increased in PTX-pretreated myocardial membranes. In the presence of 100 microM 5'-guanylyl imidodiphosphate, all agonist binding sites were converted to low-affinity sites in both control and PTX-pretreated membranes. The results suggest that Gi might regulate negatively coupling of beta-adrenoceptors to Gs.

The MAO-B inhibitor deprenyl, but not the MAO-A inhibitor clorgyline, potentiates the neurotoxicity of p-chloroamphetamine

The effect of co-administration of MAO inhibitors together with a low dose of the neurotoxic amphetamine p-chloroamphetamine (pCA) on neurotoxicity was examined. Neurotoxicity was assessed by measuring decreases in the binding of [3H]cyanoimipramine to serotonin uptake sites using quantitative autoradiography. By itself, a low dose of pCA (2 mg/kg) did not produce any alterations in radioligand binding, measured 7 days after drug administration. However, co-administration of the MAO-B selective inhibitor deprenyl (1 mg/kg) or the non-selective inhibitor pargyline (50 mg/kg) produced significant decreases in radioligand binding. Measurements of the effects of these drugs on body temperature ruled out the possibility that deprenyl and pargyline were increasing neurotoxicity by producing a drug-induced hyperthermia. In contrast to the effects of deprenyl and pargyline, co-administration of the MAO-A selective inhibitor clorgyline (1 mg/kg) did not alter binding. By themselves none of the MAO inhibitors produced neurotoxic effects. There are a number of possible explanations for these results. Administration of deprenyl or pargyline, together with pCA, itself a MAO-A inhibitor, will lead to inhibition of both MAO-A and MAO-B activities. This will likely lead to an enhanced release of dopamine and serotonin compared with the release following administration of pCA alone or pCA together with clorgyline. Elevation of the extracellular levels of either or both of these monoamines could lead to enhanced neurotoxicity. Whatever the mechanism involved, our results show that the co-administration of a type-B MAOI enhances the neurotoxic effects of pCA on serotonin neurons.

Stability of the beta-adrenergic receptor/adenylyl cyclase pathway of pediatric myocardium after brain death

Our previous work in the adult porcine model shows that brain death results in a rapid decline in left ventricular systolic function as measured by the preload recruitable stroke work method to 8% of the baseline slope within 6 hours; this process is accompanied by functional uncoupling of the beta-adrenergic receptor at the level of the adenylyl cyclase moiety within 1 hour. In contrast, the pediatric porcine myocardium displays no change in left ventricular systolic function from baseline within 6 hours of brain death. This work investigates whether the beta-adrenergic receptor/adenylyl cyclase pathway remains intact after induction of brain death in the pediatric porcine model. Thirteen 1-month-old swine (7 to 10 kg) were anesthetized and underwent median sternotomy, and baseline transmural left ventricular biopsy specimens were obtained before ligation of head vessels to induce brain death in six piglets, with the remaining seven serving as controls. Baseline left ventricular biopsy specimens were obtained just before and 1 and 3 hours after brain death or at matched time points without brain death in the control group. Myocardial tissue was then analyzed for beta-adrenergic receptor density with the use of saturation [125I]-iodocyanopindolol binding in the absence and presence of propranolol 1 mumol/L. Coupling of the beta-adrenergic receptor to its signal transduction system (stimulation of adenylyl cyclase) was tested at three levels: beta-adrenergic receptor (isoproterenol 100 mumol/L), stimulatory G protein Gs (sodium fluoride 10 mmol/L), and the adenylyl cyclase moiety itself (forskolin 100 mumol/L).(ABSTRACT TRUNCATED AT 250 WORDS)

Drug-induced regulation of [125I]iodocyanopindolol-labeled 5-hydroxytryptamine1B receptor binding sites in the central nervous system

Little is known about the regulation of 5-hydroxytryptamine1B (5-HT1B) receptors, a putative terminal autoreceptor in the central nervous system. We studied the regional responses of [125I]iodocyanopindolol ([125I]ICYP)-labeled central 5-HT1B sites to chronic treatment with 5-HT agonists and antagonists at a dose of 10 mg/kg/d IP for 30 consecutive days in the rat. In controls, there were 3.4-fold regional differences in Bmax, with a rank order of brainstem > hippocampus > cortex, striatum > spinal cord, and Kas were slightly lower in striatum and spinal cord. RU 24969 significantly reduced Bmax 23 to 63% in cortex, hippocampus, striatum, brainstem, and spinal cord without a change in Ka except for a 1.7-fold increase in cortex and spinal cord. The putative 5-HT1B agonist (m-trifluoromethyl-phenylpiperazine (TFMPP), but not [1-(3-chlorophenyl)piperazine] (m-CPP) or the 5-HT1B antagonists pindolol or quipazine, reduced the Bmax of cortical 5-HT1B sites (-16%). Chronic treatment with the 5-HT antagonists methysergide, pindolol, propranolol, ritanserin, metergoline, or methiothepin did not significantly affect striatal Bmax or Kd compared to respective vehicles. The data demonstrate significant changes in maximum number of 5-HT1B receptors in response to chronic agonist but not antagonist treatments at the dose studied.

Mediation of most atypical effects by species homologues of the beta 3-adrenoceptor

1. A wide panel of compounds acting on beta-adrenoceptors active either in mammalian heart or in rodent digestive tract and adipose tissues, were investigated for their effects on Chinese hamster ovary cells transfected with the human or murine beta 3-adrenoceptor gene. 2. The beta 3-agonists, bucindolol, CGP 12177A and pindolol exhibited the highest binding affinities; BRL 37344, LY 79771, ICI 201651 and SR 58611A presented high potencies in stimulating adenylyl cyclase; bupranolol appeared as the most efficient beta 3-antagonist. 3. This pharmacological analysis further established that the beta 3-adrenoceptor is the prototype of the adipose tissue atypical beta-adrenoceptor, since these receptors share a number of pharmacological properties which differ strikingly from those of beta 1- and beta 2-adrenoceptors: low affinities for conventional beta-adrenoceptor agonists and antagonists, high potencies for novel compounds active in adipose tissues, partial agonistic activities for several beta 1/beta 2-antagonists. 4. Although the pharmacological profiles of the human and murine beta 3-receptor were very similar, some quantitative or even qualitative differences were observed for particular compounds such as propranolol, which exhibited weak and partial agonistic effects at the human beta 3-receptors and antagonistic effects at the murine beta 3-receptors. These differences may result from key amino-acid substitutions between the human and the murine beta 3-receptor sequences, which may alter the binding site or signal processing. 5. Compounds active on atypical beta-sites of other tissues such as heart and digestive tract were also potent on the beta 3-adrenoceptor expressed in Chinese hamster ovary cells, suggesting that this receptor mediates most of the atypical properties described in various tissues, and that differences in ligand effects may result from tissue-related specificities.

Myocardial beta-adrenoceptors in pacing-induced heart failure: regulation by enalapril?

In heart failure, both the sympathetic nervous system and the renin angiotensin system play important pathophysiological roles, and the two systems may interact with each other, e.g., angiotensin II facilitating noradrenaline release. An abnormality in beta-adrenoceptor density (i.e., a decrease) occurs in clinical and pacing-induced heart failure. This observation together with the therapeutic effectiveness of converting-enzyme inhibitors in the management of patients with heart failure led to the current investigation. The aim was to explore the impact of chronic enalapril treatment on the status of myocardial beta-adrenoceptors in dogs paced (250 beats.min-1) to end-stage heart failure. Placebo or enalapril treatment (5 mg b.i.d.) commenced 1 week after the onset of ventricular pacing and continued until end-stage heart failure was reached. Myocardial beta-adrenoceptor density and affinity were assessed by radioligand binding with [125I]iodocyanopindolol. Left ventricular angiotensin II formation and noradrenaline concentration were measured. In addition, plasma renin activity and plasma noradrenaline levels were determined. The results showed that there was a significant increase in beta-adrenoceptor density following enalapril treatment compared with placebo in the heart-failure group. Enalapril did not change the beta-adrenoceptor density in the control animals. However, in both heart failure and control animals, enalapril caused an unexpected increase in Kd. Furthermore, in heart failure, enalapril caused a significant increase in myocardial angiotensin II formation. We conclude that enalapril prevents or reverses the myocardial beta-adrenoceptor abnormality seen in heart failure and promotes angiotensin II formation.

Monoamine receptors in the amygdaloid complex of the tree shrew (Tupaia belangeri)

Although it is well known that the mammalian amygdala comprises a heterogeneous complex of cytoarchitectonically and histochemically distinct nuclei, the association of these nuclei with different monoamine systems has not been described in detail. We therefore investigated the pattern of receptors for monoamines in the amygdala of the tree shrew (Tupaia belangeri). Binding sites for the alpha 2-adrenoceptor ligand (3H)rauwolscine, the alpha 1-adrenoceptor ligand (3H)prazosin, the beta-adrenoceptor ligand (125I)iodocyanopindolol, and the serotonin1A-receptor ligand (3H)8-hydroxy-2(di-n-propylamino)tetralin were visualized by in vitro autoradiography, and anatomically localized by comparing the autoradiograms to Nissl- and acetylcholinesterase-stained sections. To characterize binding of the radioligands pharmacologically, displacement experiments with different specific competitors were performed. Whereas the highest number of alpha 2-adrenergic binding sites was detected in the medial and the central nucleus as well as in the intercalated nuclei, the majority of serotonin1A binding sites was found in the magnocellular basal nucleus and the accessory basal nucleus, demonstrating a clear difference in the anatomy of the alpha 2-adrenergic and the serotonin1A receptor systems. In contrast, the pattern of alpha 1-adrenoceptor binding partially overlaps with that of both former receptor types. While the number of alpha-adrenergic and serotonin1A binding sites is relatively high in the tree shrew amygdala, there is a low number of beta-adrenergic binding sites in most nuclei. However, in the cortical nuclei, moderate to high numbers of binding sites for all radioligands are present. Therefore, according to our data on the tree shrew amygdala, which is anatomically similar to the amygdala of cats and primates, alpha 2-adrenoceptors cover primarily the medial part of the amygdaloid formation and serotonin1A-receptors predominantly occupy the basal nuclei, whereas alpha 1-adrenoceptors are present in both parts of the formation.

Responses of beta-adrenoceptor in rat soleus to phosphorus compound levels and/or unloading

Responses of beta-adrenoceptor (beta-AR) in rat soleus to gravitational unloading and/or changes in the levels of phosphorus compounds by feeding either creatine or its analogue beta-guanidinopropionic acid (beta-GPA) were studied. A decrease in the density of beta-AR (about -35%) was induced by 10 days of hindlimb suspension, but the affinity of the receptor was unaffected. Suspension unloading tended to increase the levels of adenosine triphosphate and phosphocreatine and decrease inorganic phosphate. Even without unloading, the beta-AR density decreased after an oral creatine supplementation (about -20%), which also tended to elevate the high-energy phosphate levels in muscle. However, an elevation of beta-AR density was induced (about +36%) after chronic depletion of high-energy phosphates by feeding beta-GPA (about +125%). Data suggest that the density of beta-AR in muscle is elevated if the high-energy phosphate contents are chronically decreased and vice versa. However, it may not be directly related to the degree of muscle contractile activity.